I would install two 1 1/2 inch bulkheads for overflow and two 1 inch for returns. A 1 inch overflow won't handle much water and you may end up using the sides of the tank as an overflow if you get the picture..... I would also run at least a 1500 GPH return along with a RedSea wave maker and pumps for circulation. As for the spray bar on the bottom, BAD idea! When the power goes out it works as a siphon, again you get the picture, EMPTY tank. You can install an anti siphon valve and that will work for about a week then it will have too much stuff growing in it to work anymore.
Silicone won't hold acylic, get the weld-on solvent and make a solid bond.
If you are talking about a closed loop on the spray bar why drill the tank for that? Keep the pump near the top for easy cleaning and run the PVC to the bottom for the spray bar Mag drives work well for this as you can screw the pipe right onto them. Don't forget the acrylic will make a solid layer blocking circulation so drill hundreds of hole through it. Dead spots grow CYANO.
I've probably blabbed too much already so get some more opinions and lets see what you come up with.
Tim Marvin
(512) 336-7258